As having good mechanical characteristics and workability, crystalline polypropylenes are widely used for producing films and various shaped articles in various fields. In particular, they are much used in the field of injection molding. However, since crystalline polypropylenes have poorly-balanced stiffness and impact resistance as compared with polystyrenes and ABS resins, their use is inevitably limited. If their mean molecular weight is increased, crystalline polypropylenes may have improved impact strength and therefore have well-balanced stiffness and impact resistance. However, crystalline polypropylenes having a large mean molecular weight are problematic in that their workability is lowered and, in particular, their producibility in injection molding is significantly lowered. Given this situation, it is desired to improve crystalline polypropylenes so as to have well-balanced stiffness and impact resistance without interfering with their workability.
For the purpose of obtaining crystalline polypropylenes having improved stiffness and elasticity, there is known a two-stage polymerization method of producing a mixture of polymers each having a different molecular weight. For example, JP-A-190006/1982 discloses a method of producing a mixture of polymers of which one has an intrinsic viscosity [.eta.] of from 0.6 to 1.7 dl/g and the other from 1.5 to 4.6 dl/g. JP-A-7406/1983 discloses a method of producing a mixture of polymers of which one has an intrinsic viscosity [.eta.] of from 0.6 to 3.5 dl/g and the other from 5 to 10 dl/g. However, these methods are still insufficient to improve the stiffness and elasticity of polymers.
In JP-A-356511/1992, proposed was a polypropylene that satisfies the following relation between the content, Ai (% by weight) of the constituent component having a molecular weight of from 2,000 to 26,000 and the intrinsic viscosity [.eta.] of the polymer. EQU log Ai.gtoreq.1.60-1.32.times.log [.eta.]
Although the polypropylene has improved elasticity and heat resistance, the balance of the stiffness and the impact resistance of the polymer is still not good.
On the other hand, since crystalline polypropylenes generally have poor low-temperature impact strength, they are often improved by adding thereto a rubber-type elastic material such as ethylene-propylene rubber (EPR). The impact resistance of the polymers comprising such a rubber-type elastic material may be improved, but the addition of such a rubber-type elastic material to crystalline polypropylenes is problematic in that the stiffness of the resulting polymer mixtures is inevitably lowered. Therefore, desired is a technique of improving the impact resistance of polypropylene-based resin compositions without lowering their stiffness.
Various techniques of adding ethylene-butene-1 copolymers to polypropylene-based resins have heretofore been tried. For example, known is a technique of adding a specific ethylene-butene-1 copolymer to a polypropylene-based resin to thereby make the resulting resin mixture have well-balanced stiffness and impact resistance (see JP-A-192506/1994 and JP-A-18151/1995). They say that the addition of an ethylene-butene-1 copolymer having a peak melting point of not higher than 80.degree. C. and having a low degree of X-ray crystallinity of smaller than 20% produces better results. JP-A-87478/1997 discloses a technique of adding an ethylene-butene-1 copolymer having a melting point of from 60 to 100.degree. C. to a polypropylene-based resin to thereby improve the balance of the stiffness and the impact resistance of the resulting resin mixture.
However, these polypropylene-based resin compositions comprising such an ethylene-butene-1 copolymer are still problematic in that the balance of their stiffness and impact resistance at high levels is not always satisfactory.